In the first study, we are determining if ABCs have encountered antigen and undergone somatic hypermutation and class switching. Diversity in the repertoire and the mutation frequency will be examined in old mice. This may determine if the old cells are exhausted from chronic antigen exposure. To determine what antigens induce non-responsive B cells, we will sequence heavy and light chains from single cells, clone them into expression vectors, and test for reactivity to a panel of antigens. To see if ABCs prevent affinity maturation of follicular cells, we will transfer cells into recipient mice, immunize, and measure affinity. In the second study, uracil in the genome can result from misincorporation of dUTP instead of dTTP during DNA synthesis, and is primarily removed by uracil DNA glycosylase (UNG) during base excision repair. Telomeres contain long arrays of TTAGGG repeats and may be susceptible to uracil misincorporation. Using model telomeric DNA substrates, we showed that the position and number of uracil substitutions of thymine in telomeric DNA decreased recognition by the telomere single-strand binding protein, POT1. In primary mouse hematopoietic cells, uracil was detectable at telomeres, and UNG deficiency further increased uracil loads and led to abnormal telomere lengthening. In UNG-deficient cells, the frequencies of sister chromatid exchange and fragility in telomeres also significantly increased in the absence of telomerase. Thus, accumulation of uracil and/or UNG deficiency interferes with telomere maintenance, thereby underscoring the necessity of UNG-initiated base excision repair for the preservation of telomere integrity.